Non-volatile memory arrays retain information in the absence of power. Examples of non-volatile memory arrays include flash memory, erasable programmable read-only memory (EPROM), and electrically-erasable programmable read-only memory (EEPROM). Non-volatile memory arrays may be used in a variety of electronic devices, such as microcontrollers, to provide storage capability.
A typical non-volatile memory cell may be formed using a double-polysilicon structure. This structure includes two layers of polysilicon. The upper polysilicon layer forms a control gate and word lines, and the lower polysilicon layer forms a floating gate. The floating gate, which is insulated by a gate-oxide layer, is capable of storing charge. The presence or absence of charge on the floating gate determines the digital value of the cell, with the presence of charge yielding a logical low and the absence of charge yielding a logical high.
In order to place charge on the floating gate, a relatively high voltage is applied to the control gate and the drain. As a result of this excitation, some of the electrons in the channel acquire enough energy to be transported from the channel through the gate-oxide layer and into the floating gate. Once the high voltage is removed, the electrons do not have enough energy to cross through the barrier of the gate-oxide layer. Thus, the charged cell may retain the charge on the floating gate indefinitely.
The negative charge captured in the floating gate makes the channel more positive and, thus, less conducting. As a result, the threshold voltage at the bit is higher for a charged cell than for an uncharged cell. Consequently, for a given voltage applied to the control gate, the non-volatile memory cell will conduct if the floating gate has no stored charge and will not conduct if the floating gate has stored charge. Therefore, a logical low or high is provided by the non-volatile memory cell based on whether or not the cell conducts at a given threshold voltage.
In order to remove charge from the floating gate, the non-volatile memory cell is irradiated with ultraviolet light. The ultraviolet light provides the stored electrons on the floating gate with enough extra energy to cross the barrier of the gate-oxide layer. However, this is a relatively time-consuming process. To reduce the time required to complete this process, the temperature of non-volatile memory arrays may be elevated during erasure to add to the energy obtained from the ultraviolet light. The use of elevated temperatures, however, has led to a high incidence of breakage of the relatively thin wafers on which the non-volatile memory arrays are fabricated.